1. Technical Field
The present invention relates to a material testing machine, and more particularly, to an electrohydraulic servo-controlled material testing machine.
2. Related Art
An electrohydraulic servo-controlled material testing machine is known which is comprised of a controller for supplying, via a servo amplifier, an electric signal input to an electrohydraulic servo valve provided between a hydraulic power source and a hydraulic actuator such as hydraulic cylinder, the electric signal input varying in accordance with a target displacement of a movable part of the actuator. Typically, the quantity of fluid output from the servo valve varies in response to the electric signal input, and the movable part of the hydraulic cylinder is displaced at a speed proportional to the fluid quantity, whereby a load is applied to a test piece held between the cylinder movable part and a main body of the testing machine. An actual displacement of the cylinder movable part is detected and is supplied as a feedback signal to the controller. Under the control of the controller, a feedback control is carried out to cause the actual displacement to close to a target displacement.
In this specification, the term "displacement of a test piece" indicates the displacement of one end of the test piece coupled to a movable part of an actuator which displacement is caused by the displacement of the movable part of the actuator. In a material testing machine of a type provided with two actuators whose movable parts hold a test piece therebetween are typically displaced in opposite directions, the term "displacement of the test piece" indicates the sum of displacements of opposite ends of the test piece caused by the displacement of the two movable parts of the actuators. That is, the term "displacement of the test piece" indicates deformation of the test piece caused by the displacement of the movable part(s) of the actuator.
The term "servo system" indicates a system mainly comprised of an actuator, a servo amplifier, and a servo valve. The term "control system" indicates a system mainly comprised of a servo system and a controller for controlling the operation of the servo system. The term "control loop" or "feedback control loop" indicates a loop mainly comprised of a servo system, a controller, and a test piece. Moreover, the term "force control system" indicates a control system for carrying out a feedback control with use of an actual force applied to the test piece, as the controlled variable, whereas the term "displacement control system" indicates a system for executing a feedback control using, as a controlled variable, an actual displacement of the test piece. The term "load" indicates a broadly defined load which includes a force applied to the test piece and generally referred to as a load, and which also includes the displacement of the test piece. If that the actual load or the actual displacement is referred to as a controlled variable, the term "control objective value" or "control target value" indicates a target load.
The material testing machine sometimes carries out a fatigue test in which mechanical properties of a test piece are measured while the load given to the test piece is periodically changed. For example, the actual force applied to and the actual displacement of the test piece are measured while the target load, e.g., the target force applied to the controller of the testing machine periodically changes in a specific pattern such as sinusoidal a wave, triangular wave, rectangular wave or ramp wave. On this occasion, under the feedback control of the control system, the actual force generally changes following the variable target force. As the repetitive frequency of the target force, inversely proportional to the cycle of the target force changing pattern, becomes higher, the followingness of the actual force to the target force is deteriorated, causing the testing accuracy to be lowered.
At the occasion of a fatigue test, a testing waveform indicative of the target force which periodically varies in specific patterns is given to the servo control system of the testing machine. The amplitude and repetition cycle of the testing waveform are set so as to be suited to the kind or size of the test piece or the kind or purpose of material testing. For instance, in the case of a fatigue test, the repetitive frequency of the testing waveform varies, depending on the testing purpose, within a range from a very low frequency in the order of 0.01 Hz to a certain frequency in the order of 100 Hz which is an operational upper limit of hydraulic servo system.
A waveform generator for generating, in an analog fashion, various types of testing waveform, especially a testing waveform which varies at very low frequency, is generally extremely high in cost and is complicated in construction. Thus, various attempts have been made to generate a variety of testing waveforms in a digital fashion. However, this requires a memory of large capacity for storing therein waveform data based on which various types of testing waveform are generated, or requires considerably complicated operation for generating a testing waveform with high accuracy.